Converters Having a Plurality of Channels
A converter having a plurality of channels comprises a transformer having a primary winding and at least one secondary winding. The transformer may comprise a plurality of secondary windings, with each secondary winding corresponding to one or more channels. The transformer may also have only one secondary winding, corresponding to all of the channels of the converter.
FIG. 1 is a schematic diagram of an example of a prior art converter circuit having a plurality of channels.
In addition to the transformer 2 having a primary winding 3, a main secondary winding 4, and an auxiliary secondary winding 5, the converter also includes first switch means. The first switch means may comprise a switch T0, e.g. a transistor, connected to the primary winding 3.
The switch T0 serves to chop a substantially continuous or direct current (DC) voltage (not shown) in compliance with a first squarewave periodic signal Vc1 to form an input signal Ve. The switch T0 is controlled by a primary control device using pulse width modulation (PWM) to control the width of the pulses of the first squarewave periodic signal Vc1.
The converter 1 also comprises demagnetization means (not shown), e.g. an active clamp, or making use of resonant reset, or indeed constituted by a demagnetization winding. The average over one period of the input signal Ve is substantially zero. The input signal Ve presents a freewheel phase corresponding to those times during which the first squarewave periodic signal Vc1 is substantially zero, and a positive sequence phase substantially during the pulses in the first squarewave periodic signal Vc1.
A first output signal V1 on a main channel 7 is of an amplitude that is substantially proportional to the amplitude of the input signal Ve. A rectifier and smoothing circuit 6 serves to obtain a main DC signal V1′ from the first output signal V1. The voltage value of the main DC signal V1′ is a function of the duty ratio of the first squarewave periodic signal Vc1.
Converters with Chopping in Cascade
The converter presents chopping in cascade, i.e. in addition to the first switch means, the converter includes second switch means at the input of one of the channels, and in this example it includes an auxiliary switch T1. Thus, the device that makes it possible to obtain an auxiliary DC signal V2′ from a second output signal V2 on an auxiliary channel 8 is practically identical to the rectifier and smoothing circuit 6 with the exception that the auxiliary switch T1 enables the second output signal V2 to be chopped in compliance with a second squarewave periodic signal Vc2. The second output signal V2 has an amplitude that is substantially proportional to the amplitude of the input signal Ve.
By way of example, the second squarewave periodic signal Vc2 presents pulses of width smaller than the width of the pulses in the first squarewave periodic signal Vc1, the pulses in the second squarewave periodic signal Vc2 terminating for example at the same time as the pulses in the first squarewave periodic signal Vc1. A chopped signal V2r thus presents a non-zero voltage value only during the pulses of the second squarewave periodic signal Vc2. The chopped signal V2r thus presents a waveform that is substantially that of a squarewave periodic signal, with the pulses in the chopped signal being of a width that is smaller than the width of the pulses in the first squarewave periodic signal Vc1.
The auxiliary DC signal V2′ is then obtained by rectifying and smoothing the modulated signal V2r. The value of the auxiliary DC signal V2′ thus depends in this example on the delay between the rising fronts of the second squarewave periodic signal Vc2 and the rising fronts of the first squarewave periodic signal Vc1.
Current Limitation
The converter may include a circuit for limiting current (not shown), in order to protect components.
Current limitation is frequently performed at the secondary, i.e. the current in the main channel is measured, e.g. by using a measurement resistor known as a “shunt”. The voltage across the terminals of the measurement resistor is stored, processed, and forwarded via an optical coupler to the primary control device. The primary control device modifies the value of the duty ratio of the pulses in the first squarewave periodic signal Vc1 as a function of the value of the current as measured in the main channel. Such a loop serves to regulate the current in the main channel with a relatively high degree of accuracy.
Similarly, the current in the auxiliary channel is measured and the value of the duty ratio of the second squarewave periodic signal Vc2 is adjusted or modified as a function of the measured current value.
Current limiting may also be performed on the primary. The current passing through the primary winding, known as the primary current, is measured. Such a current limiter circuit has only a single measurement resistor and is consequently more compact and less expensive than a current limiter circuit acting on the secondary.
The measured value of the primary current gives overall information relating to a plurality of channels. The value of the duty ratio of the pulses in the first squarewave periodic signal Vc1 is adjusted as a function of the value of the primary current.
Nevertheless, if one of the channels should be open-circuit, for example, then the current flowing in that channel will be zero. Given the way energy is converted in a transformer, in particular when the powers for each of the channels are very different, it can happen that current limitation occurs only when the current in the remaining channel is relatively high. Current limitation can then be ineffective in protecting components.